Current electric appliances demonstrate a tendency to have a reduced weight and a smaller size, which leads to development of lithium ion secondary batteries having a high energy density. Further, lithium ion secondary batteries are used in more various fields, and thus are desired to have improved battery characteristics. In particular, the safety and battery characteristics of lithium ion secondary batteries will become more and more important factors when the batteries are put in use for automobiles.
However, lithium ion secondary batteries are less sufficiently safe when, for example, they are overcharged, internally short-circuited, or penetrated by a nail. Thus, much safer batteries are required for the use in automobiles. In addition, the voltage of a battery needs to be much higher so as to increase the capacity of the battery for the use in automobiles.
When a lithium ion secondary battery is supplied with a voltage, it may generate gas due to electrochemical oxidation or reduction in some cases. When gas is generated, it increases the pressure inside the battery, so that the battery may burst or the charge and discharge efficiency may deteriorate. Such generation of gas is likely to occur especially when the battery is overcharged or used under high voltage.
In order to improve the safety, charge characteristics, and electric properties of electrochemical devices, such as lithium ion secondary batteries, various methods are proposed.
Patent Literature 1 discloses that a non-aqueous electrolyte solution secondary battery comprising a fluorinated acyclic carbonate suppresses generation of gas due to electrolysis, and thus is highly safe.